Hydraulic tubing is used in a variety of machines and manufacturing facilities to carry hydraulic fluid and other liquids at high pressures and sometimes at elevated temperatures. Typically hydraulic tubing is made from low carbon steel or stainless steel and is usually sold in lengths 20 feet long. Hydraulic tubing typically has an outside diameter for from 3/16 to 1-¼ inches and a wall thickness of from 0.035 to 0.120 inches. When sold in metric sizes hydraulic tubing typically has an outside diameter for from 4 to 28 mm and a wall thickness of from 0.50 to 3.00 mm.
Hydraulic tubing made from low carbon steel is zinc plated on the outside surface but no coating is applied to the inside surface. Sometimes caps are placed over the ends of the tubes for storage. While the tubes are in storage rust frequently forms on the inside surface of the tubes. This can happen even if the ends of the tubes are capped. If the rust is not removed before the tubes are put in service, rust particles may migrate from the surface of the tube into fluid passing through the tube thereby contaminating the fluid. Rusting is not a problem with stainless steel tubing, but those tubes are more expensive than low carbon steel tubes.
It is well known in the art to provide coatings on metal surfaces to resist rust. Such coatings include latex paints, oils, polymer coatings, phosphate coatings, and electroplating. Nevertheless, the art has only electroplated the outside of low carbon steel hydraulic tubes with the inside surface being bare metal. This may have been done under the mistaken belief that rusting did not occur on the inside surface of the low carbon steel hydraulic tubes or that any rusting that did occur was insignificant. However, I have observed that significant rusting can occur when conventional low carbon steel hydraulic tubes are stored for several months, particularly, if the tubing is exposed to rain or snow while in storage. I have also found that such rusting can adversely affect the performance of hydraulic fluid passing through such rusted low carbon steel hydraulic tubes. Furthermore, capping the ends of the tubes does not solve the problem. Consequently, there is a need for low carbon steel hydraulic tubes that will not rust while in storage.